Method of growing semiconductor crystal

ABSTRACT

SiC is a very stable substance, and it is difficult to control the condition of a SiC surface to be suitable for crystal growth in conventional Group III nitride crystal growing apparatuses. This problem is solved as follows. The surface of a SiC substrate  1  is rendered into a step-terrace structure by performing a heating process in an atmosphere of HCl gas. The surface of the SiC substrate  1  is then treated sequentially with aqua regia, hydrochloric acid, and hydrofluoric acid. A small amount of silicon oxide film formed on the surface of the SiC substrate  1  is etched so as to form a clean SiC surface  3  on the substrate surface. The SiC substrate  1  is then installed in a high-vacuum apparatus and the pressure inside is maintained at ultrahigh vacuum (such as 10 −6  to 10 −8  Pa). In the ultrahigh vacuum state, a process of irradiating the surface with a Ga atomic beam  5  at time t 1  at temperature of 800° C. or lower and performing a heating treatment at 800° C. or higher is repeated at least once. The temperature is then set to the growth temperature of an AlN film, and the SiC substrate surface  3  is initially irradiated with —Al atoms  8   a  in ultrahigh vacuum state, followed by the feeding of N atoms  8   b.

TECHNICAL FIELD

The present invention relates to a semiconductor crystal growingtechnology, and more particularly to a technique for growing Group IIInitride crystals on an SiC surface.

BACKGROUND ART

Hexagonal SiC has a very high heat conductivity, and both electricallyconductive and insulating substrates are available. Its lattice constantand coefficient of thermal expansion are relatively close to those ofAlN and GaN-based Group III nitrides. Another characteristic ofhexagonal SiC is that it is a hexagonal crystal and possessespolarities, as to Group III nitrides.

There are high expectations for the realization of a technology forgrowing high-quality crystals of Group III nitrides on SiC forapplications relating to a buffer layer for the formation of a GaN-baseddevice structure on an SiC substrate, or relating to Group IIInitride/SiC heterojunction devices. It has been difficult to grow ahigh-quality Group III nitride layer on SiC because of the mismatch ofthe stacked structure of SiC and Group III nitrides in the c-direction,or the so-called polytype mismatch. Namely, 4H—SiC and 6H—SiC, which arerepresentative of hexagonal SiC, have structures with 4- and 6-monolayerperiods, respectively, in the c-axis direction, while AlN and GaN, whichare Group III nitrides, have 2-monolayer periods in the c-axis directionin a structure referred to as the wurtzite structure.

In order to solve this problem, it has been proposed to make the SiCsubstrate surface a flat plane without any steps, or to control theheight of the steps on the SiC substrate surface to be common multiplesof the stacking periods of SiC and the Group III nitride. For example, atechnology has been proposed whereby a SiC substrate surface issubjected to HCl gas etching so as to form a SiC surface with theaforementioned features, which is followed by the growing of an AlNlayer (see Non-patent Document 1: Norio Onojima, Jun Suda, and HiroyukiMatsunami, “Molecular-beam epitaxial growth of insulating AlN onsurface-controlled 6H—SiC substrate by HCl gas etching,” Applied PhysicsLetters, Vol. 80, No. 1, (2002) p. 76-78, for example).

DISCLOSURE OF THE INVENTION

While it is expected that the inconsistency in stacked structures can beresolved by the aforementioned method, there are two additional problemsin the crystal growth of Group III nitride on SiC surfaces. One is thatthe SiC surface is chemically and thermally stable, and it is difficultto control the SiC surface condition unless a high-temperatureenvironment, such as one exceeding 1200° C., is used. Suchhigh-temperature environment, however, is difficult to realize viaconventional Group III nitride growth apparatuses.

The second problem is that because the interface of crystal growth is aninterface of substances with different chemical bonds, namely IV-IV andIII-V, there would be either an excess or a lack of electrons in theformation of covalent bonding, such as in the cases of IV-III or IV-V,at the interface. As a result, the interface could be destabilized andthe mode of crystal growth tends to be that of three-dimensional islandgrowth. Thus, it has been difficult to achieve high-quality Group IIInitride crystal growth.

It is an object of the invention to provide a technique for growinghigh-quality Group III nitride crystals on SiC by controlling thesurface of SiC and/or through sequence control at the beginning ofgrowth.

In one aspect, the invention provides a crystal growing methodcomprising the steps of: forming a step-terrace structure that is flaton the atomic level on a SiC surface and then removing an oxide filmfrom the surface; and performing at least one cycle of a process ofirradiation of Si or Ga under high vacuum and then heating, and thengrowing a Group III nitride.

In this crystal growing method, the SiC surface is made flat and cleanwhile the steps thereon are controlled, and then the process ofirradiating the surface with Si or Ga. Then it is heated at least onceunder high vacuum, followed by the growth of a Group III nitride. As aresult, the amount of oxygen remaining on the SiC surface can beminimized, and a SiC/Group III nitride with good interface can beformed.

The invention also provides a crystal growing method comprising thesteps of: forming a flat and clean SiC surface; and growing a Group IIInitride under high vacuum, wherein a Group III element is fed beforenitrogen is fed.

In this method, because nitrogen is fed after the Group III element hasbeen fed onto the clean SiC surface, the formation of a nitride layer oran excess Si—N bond due to the reaction of Si and nitrogen on the SiCsurface can be prevented. Further, the preceding supply of the Group IIIelement onto the SiC surface helps to reduce the instability at theinterface, so that two-dimensional layer-by-layer growth can be realizedwith good reproducibility.

In another aspect, the invention provides a crystal growing methodcomprising the steps of: forming a flat and clean SiC surface; growing aGroup III nitride under high vacuum, wherein a surface control elementfor forming a surface control layer for controlling the mode of crystalgrowth of said Group III nitride on the SiC surface is fed; and feedinga Group III element and nitrogen, followed by the termination of thefeeding of said surface control element. Preferably, the surface controlelement is Ga or In.

By thus feeding the Group III element and nitrogen after the feeding ofthe surface control element, the formation of a nitride layer of Si orthe like on the SiC surface can be prevented regardless of the order offeeding of the Group III element and nitrogen. As a result, a goodinterface can be formed and the need to control the order of feeding ofthe Group III element and nitrogen can be eliminated.

In yet another aspect of the invention, the invention provides a crystalgrowing method comprising the steps of: controlling the SiC surface toacquire a step-terrace structure that is flat on the atomic level; andremoving an oxide film from the surface in an atmosphere of reducedoxygen partial pressure and then growing a Group III nitride. In thismethod, the re-adsorption of oxygen after the step of removing thesurface oxide film can be prevented, so that a good-quality singlecrystal can be formed even without performing a surface oxide-filmremoving process via Ga irradiation or heating prior to the growth ofthe Group III nitride.

In yet another aspect, the invention provides a stacked structurecomprising: an SiC layer; a Group III nitride layer; and Ga atoms or Inatoms remaining between said SiC layer and said Group III nitride layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the steps of growing a Group III nitridecrystal on SiC in accordance with an embodiment of the invention.

FIG. 2(A) shows a sequence chart regarding the feed timing of a growthelement (which is controlled by turning on or off a shutter or the likein the case of MBE) before and after the crystal growth of a Group IIInitride on SiC in accordance with a first embodiment of the invention.

FIG. 2(B) shows a sequence chart illustrating a variation of the crystalgrowing method of FIG. 2(A).

FIG. 3(A) shows the result of measuring the electron diffractionintensity observed in situ following the start of growth of an AlNlayer.

FIG. 3(B) shows measurement results obtained when the growth temperatureof the AlN layer was lowered.

FIG. 4 shows the result of measuring an X-ray rocking curve in anasymmetric plane (01-14) in a sample in which a hydrofluoric acidprocess and a Ga atomic beam irradiation process were not carried out,in a sample in which the Ga atomic beam irradiation process was notcarried out, and in a sample in which both the hydrofluoric acid processand the Ga atomic beam irradiation process were performed.

FIG. 5 shows the steps for crystal growth in accordance with a secondembodiment of the invention.

FIG. 6 shows a sequence chart regarding the timing (which is controlledby turning on or off of a shutter, for example, in the case of MBE) offeeding of a growth element before and after the growth of a Group IIInitride crystal.

FIG. 7 shows the steps of crystal growth in accordance with a thirdembodiment of the invention.

FIG. 8 shows a sequence chart regarding the timing (which is controlledby turning on or off of a shutter, for example, in the case of MBE) offeeding of a growth element before and after the growth of a Group IIInitride crystal.

FIG. 9 shows a crystal growing method in accordance with a fourthembodiment of the invention.

FIGS. 10(A) to (D) shows examples of structures grown by the crystalgrowing method according to the first or third embodiment of theinvention.

FIG. 11(A) shows an example of the cross-sectional structure of a MISFETin which the band discontinuity and the difference in electron affinitybetween SiC and AlN are utilized and in which a two-dimensional electrongas layer is used as a channel layer.

FIG. 11(B) shows an energy band structure in a SiC-AlN junctionstructure.

FIG. 12 shows a cross section of a laser structure, illustrating anexample in which the crystal growing technique according to anembodiment of the invention is applied to an optical device.

FIG. 13 shows a cross section of a GaN/SiC-based HBT, illustrating anexample in which the crystal growing technique according to anembodiment of the invention is applied to an HBT (heterojunction bipolartransistor).

BEST MODES FOR CARRYING OUT THE INVENTION

The technique for growing Group III crystals on SiC according toembodiments of the invention will be described with reference to thedrawings. The term “SiC surface” herein includes both the surface of aSiC substrate and the surface of a SiC layer deposited on differenttypes of materials, such as a Si substrate and a sapphire substrate. Theterm “SiC substrate” also includes substrates on the surface of whichSiC exists.

The term “Group III nitride” refers to nitrides containing N and atleast one of Group III elements B, Al, Ga, or In. The nitrides mayinclude elements other than N as the Group V element. They may alsoinclude not only compounds such as GaN or AlN, but also mixed crystals,such as Al_(x)In_(y)Ga_(1-x-y)N.

Initially, the crystal growth technique according to a first embodimentof the invention will be described with reference to the drawings. FIG.1 schematically shows a process chart of the steps for growing a crystalof a Group III nitride on SiC according to the first embodiment. FIGS.2(A) and (B) show growth sequence charts illustrating the relationshipbetween the grown-element feed time (which is controlled by theswitching on and off of the shutter, for example, in the case of MBE)and growth temperature before and after the growth of crystal of theGroup III nitride.

In the following, the step of growing a crystal is described withreference to FIGS. 1 and 2. Initially, a washed 4H—SiC (0001)_(Si)substrate 1 (which may alternatively be comprised of 6H—SiC) is prepared(which is either a just substrate or a substrate with an offset angle ofnot more than 15° in an arbitrary crystal direction. With greater offsetangles, problems regarding the proximity of the steps or the formationof a facet would arise, which would then require the consideration ofanother crystal growth mechanism). The substrate is then subjected toheat treatment in an atmosphere of HCl gas diluted with hydrogen gas ina quartz furnace (with a flowrate of hydrogen gas, which is the carriergas, of 1 slm and a flowrate of HCl gas of 3 sccm) at 1300° C. for 10minutes, for example. The etch rate of SiC under these conditions isapproximately 0.3 em/h. Through such a gas etching process, the surfaceof the SiC substrate is formed into a step-terrace structure consistingof terraces that are flat on the atomic level and steps, as mentionedabove. The width of the terraces depends on the plane orientation of thesubstrate (off-angle). For example, if there is an offset angle ofapproximately 0.2° with respect to the (0001)_(Si) plane, the terracewidth would be on the micrometers order. The height of the steps can becontrolled to result in 4 monolayers by adjusting the angle ofinclination of the SiC substrate surface, the crystal orientation in thedirection of inclination, and gas etching conditions. Instead of gasetching, or following gas etching, homoepitaxial growth of SiC may becarried out (as shown by layer 2 in FIG. 1(A)). It is also possible toform a surface with the aforementioned step height by adjusting thegrowth conditions during homoepitaxial growth. Carrying outhomoepitaxial growth, which allows the formation of a SiC layer withbetter quality than the substrate, is useful for the formation of abetter-quality Group III nitride layer or for the preparation of adevice in which a Group III nitride/SiC interface is used.

Following the gas etching, the SiC substrate is taken out to theatmosphere. When the thus taken-out substrate was evaluated using anatomic force microscope, it was revealed that the substrate surface hada step-terrace structure, with the terrace surface being flat on theatomic level. The height of the steps was equal to 4 monolayers. It canbe seen that by providing the surface of the SiC substrate with theaforementioned structure prior to the growth of the Group III nitride,one of the conditions for growing a high-quality structure can besatisfied.

Then, as shown in FIG. 1(B), the surface of the SiC substrate 1 takenout into the atmosphere was treated with aqua regia, hydrochloric acid,and hydrofluoric acid sequentially. By the hydrofluoric acid process, aminute amount of silicon oxide film formed on the surface of the SiCsubstrate 1 can be removed. On the surface of the substrate, there isformed a clean surface 3 of SiC. When the thus treated substrate wasevaluated using an atomic force microscope, a step-terrace structure wasobserved on the surface of the SiC substrate 1. It was thus observedthat the step-terrace structure on the surface does not change and canbe maintained even after the foregoing chemical treatment. When thesurface was further analyzed by X-ray photoelectron spectroscopy (XPS),it was revealed that the amount of oxygen on the surface was greatlyreduced by the treatment involving aqua regia, hydrochloric acid, andhydrofluoric acid. However, it was also confirmed that a minute and yetsignificant amount of oxygen remained.

As shown in FIG. 1(C) and FIG. 2(A), the SiC substrate 1 was installedin a high-vacuum apparatus, such as an MBE (molecular beam epitaxy)apparatus, and the apparatus was maintained in a ultra-high vacuum state(on the order of 10⁻⁶ to 10⁻⁸ Pa, for example). In the ultrahigh vacuumstate, an irradiation of a Ga atomic beam 5 was started at time t1 attemperature of not 800° C. or higher (600° C. in the illustratedexample). The temperature was then increased to more than 800° C. (1000°C. in the illustrated example) between time t2 and t3, and the hightemperature was maintained in the period t3 to t4.

The process of heating and maintaining a certain temperature was carriedout at least once (twice in the illustrated example) and preferablyrepeated three times or more. Preferably, the Ga irradiation isinterrupted during heating (as indicated by the solid line). However,similar results would be obtained by continuing the Ga irradiationduring the period t3 to t4 and then terminating at t4 (as indicated bythe broken line). In this case, by providing intervals Δt1 and Δt2 fromthe interruption of Ga until the lowering of temperature, time may beprovided for the separation of Ga from the surface. Alternatively, Gamay be continuously fed until t8 or t8−Δt2. Thereafter, the temperatureis lowered from 1000° C. to 600° C. between t4 and t5, and Ga is againirradiated at t5 and the temperature is maintained until t6. Then, theirradiation of Ga is interrupted at t6 and the temperature is increasedto 1000° C. until t7. The irradiation of Ga may similarly be interruptedat time t8, which is at the end of the period in which the temperatureis maintained at 1000° C., without interrupting at time t6. Thereafter,the temperature is lowered to 900° C., for example, between t8 and t9,and Al and N are fed simultaneously at time t10, when the growth of AlNbegins.

Instead of the Ga atomic beam 5, or in addition to the Ga atoms, a Siatomic beam may be shone. As a result of surface analysis using XPS, itwas shown that the amount of oxygen on the surface after the Gairradiation and heating process was below the measuring limit of themeasuring apparatus. Thus, through the Ga irradiation and the subsequentheating process, it became possible to virtually completely remove theoxygen on the surface that had not been completely removed by thehydrofluoric acid process or that had been adsorbed via the atmospherewhile the substrate was being mounted on the MBE apparatus following thehydrofluoric acid process.

Regarding the details of the AlN growing step, the temperature is set tothe growth temperature for the AlN film (Ts=400° C. to 1100° C. forexample; 900° C. in the illustrated example), and Al atoms 8 a and Natoms 8 b are fed to the surface of the SiC substrate in a ultrahighvacuum state (such as 10⁻⁶ to 10⁻⁸ Pa), as shown in FIG. 1(D). Thedegree of vacuum during growth is determined by the balance between theamount of N atoms that are fed and the evacuating performance of thegrowing apparatus. Under the general growth conditions, the degree ofvacuum would be on the order of 10⁻² to 10⁻⁴ Pa. The N atoms 8 b werefed to the substrate surface by the rf-MBE method using rf-plasmaexcited active nitrogen, for example. From this point in time on, an AlNlayer 7 can be grown on the SiC substrate 1, as shown in FIG. 1(D).

FIG. 3 shows the result of measurement of electron diffraction (RHEED)observed in situ following time t10, when the growth of the AlN layer 7began. FIG. 3 also shows the changes in the RHEED intensity in a samplein which AlN was grown without the Ga atomic beam irradiation of FIG.1(C), and in another sample in which the hydrofluoric acid process ofFIG. 1(B) were additionally omitted, depending on the thickness of theAlN film grown.

As shown in FIG. 3, in the electron diffraction intensity observed insitu after time t3 when the growth of the AlN layer 7 began, nooscillation of RHEED was observed in the sample in which thehydrofluoric acid process and the Ga atomic beam irradiation process hadnot been performed, and in the sample in which the Ga atomic beamirradiation process had been omitted. Namely, in contrast to thedominance of the three-dimensional island growth in the AlN layer 7, inthe sample that had been subjected to the hydrofluoric acid process andthe Ga atomic beam irradiation process, oscillation of RHEED wasobserved. It was therefore confirmed that the AlN layer was grown in alayer-by-layer mode (i.e., layered two-dimensional growth instead of theisland-like three-dimensional growth) on the SiC surface.

The continuation of the RHEED oscillation varies depending on the growthconditions. For example, continuation of oscillation over more thanseveral tens of periods can be confirmed in the growth at lowertemperatures. Atomic force microscopic observation of the step structureof the surface where the AlN layer was grown revealed that theattenuation of oscillation at higher temperatures was due to thetransition of growth mode from the layer-by-layer growth to the stepflow growth. It is noted, however, that the step flow growth is also atwo-dimensional growth and is therefore as preferable as thelayer-by-layer growth in terms of crystal growth for achieving highercrystal quality. FIG. 3(B) shows the RHEED oscillation in a case wherethe Group III nitride crystal growth temperature was lowered to about600 to 700° C. The figure clearly shows the appearance of the RHEEDoscillation over 20 or more periods, thus indicating that thelayer-by-layer growth can be maintained over a long time by lowering thecrystal growth temperature. However, at lower temperatures such as 400°C. or below, the crystallinity greatly deteriorates due to the migrationof atoms or an insufficient re-separation of excess raw material. Thus,temperatures of 400° C. or higher are required if high-quality AlN is tobe obtained.

FIG. 4 shows the result of measuring the X-ray rocking curve in thenon-symmetrical plane (01-14) with regard to the above-mentioned threekinds of samples, where −1 is equivalent to 1 to which the bar sign isgiven at the top. The vertical axis shows the half-value width of theX-ray rocking curve, and the horizontal axis shows the film thickness ofthe AlN layer. The greater the half-value width of the rocking curve,the more fluctuations there are in the crystal plane, namely, the poorerthe quality of crystal. As shown in FIG. 4, in the sample in whichneither the hydrofluoric acid process nor the Ga atomic beam irradiationprocess had been performed, the half-value width is 1000 to 3000seconds, while in the sample in which only the Ga atomic beamirradiation process had not been performed, the half-value width is 800to 2000 seconds. In contrast, in the sample in which the hydrofluoricacid process and the Ga atomic beam irradiation process had beenperformed, the half-value width of the X-ray rocking curve is greatlyreduced to 300 to 500 seconds. It is therefore seen that the fluctuationof the crystal plane is very small in the latter and that thecrystallinity is very good. This is believed to be the effect of thetwo-dimensional nuclei of AlN having fused laterally in the initialstages of growth, thereby growing in layers, as a result of realizationof the layer-by-layer growth, instead of the growth of the AlN layer inthe form of individual three-dimensional islands.

Thus, in accordance with the first embodiment of the invention, after aSiC surface and a surface with common multiples of the total number ofthe molecules in the SiC surface and the AlN layer that is depositedthereon are prepared, the AlN layer is grown, whereby AlN can be formedon the SiC surface by layer-by-layer growth or step-flow growth(two-dimensional growth). As a result, defects are less likely to beintroduced as compared with the conventional case of three-dimensionalisland growth, such that high-quality crystal growth can be achieved.During the AlN crystal growth, N may be intermittently fed while an Albeam is shone.

There are many steps on the SiC surface prior to the AlN growth. Theheights of all of the steps preferably correspond to the least commonmultiples of the total number of the molecules of SiC and AlN. However,it is known that significant effect for achieving higher quality of theAlN crystal can be obtained even if some steps do not have such heightas long as roughly more than half of the steps have such height. It israther more important for achieving higher quality of the AlN crystalthat the chemical processes and the Ga atomic beam irradiation andheating processes are performed appropriately and that the growthtemperature of AlN and the feed ratio of Al and N are appropriately set.

With reference to FIG. 2(B), a method of crystal growth according to avariation of the first embodiment is described. As opposed to FIG. 2(A),in the crystal growing method shown in FIG. 2(B), the Ga irradiation isinitiated at time t1 while the substrate temperature is maintained at800° C. When Ga is shone at 800° C., the rate of feeding of Ga to theSiC surface and the rate of separation of Ga from the SiC surfacesubstantially correspond, whereby a kind of equilibrium is achieved. Inthis state, Ga reacts with the Si oxide film remaining on the SiCsurface and evaporates from the substrate surface in the form of Gaoxide with a relatively high vapor pressure. As a result, the Si oxidefilm on the SiC surface can be eliminated.

At time t2, or prior thereto, the irradiation of Ga is interrupted, andthe temperature is increased up to 1000° C. between time t2 and t3. Thetemperature of 1000° C. is then maintained until t4. Between time t4 tot5, the temperature is lowered to 900° C., which is the growthtemperature, and at a certain time to, Al and N are simultaneously fed,whereby AlN is grown. In this method, the silicon oxide film on the SiCsurface can be eliminated prior to the growth of AlN.

Alternatively, in FIG. 2(B), Ga may be fed until Δt3 before t4, and thenGa may be removed in the period Δt3.

Hereafter, a crystal growth method according to a second embodiment ofthe invention is described with reference to the drawings. FIG. 5 showsthe steps of the crystal growing method according to the presentembodiment. FIG. 6 shows a sequence chart regarding the timing offeeding of growth elements (which is controlled by turning on and off ofa shutter, for example, in the case of MBE) before and after the crystalgrowth of a Group III nitride.

In the following, the crystal growth step in the present embodiment isdescribed in detail with reference to FIGS. 5 and 6. As shown in FIG.5(A), initially a washed 4H—SiC(0001)_(Si) substrate 11 is prepared.Then, a clean surface 15 is formed on the surface of the substrate 11,as in the case of the above-described first embodiment. Thereafter, asshown in FIG. 5(B) and FIG. 6, the Al irradiation is carried out firstat time t11. At time t12 (t12−t11=10 s, for example), when the surfaceof the SiC substrate 11 is substantially entirely covered with Al,rf-plasma excited active nitrogen 20 is fed, as shown in FIG. 5(C). Atthis time t12, the growth of a AlN layer 23 begins. At t13, the feedingof Al and N is terminated, whereby the growth of the AlN layers 23stops. As a result, the possibility of the N atoms (rf-plasma excitedactive nitrogen) 21 directly reacting with the SiC surface 15 isreduced, so that the formation of the unwanted SiN layer, or an excessSi—N bond on the SiC surface 15 can be prevented and therefore the stateof interface between the SiC surface 15 and the AlN layer 23 can befavorably maintained. The Al atoms 17 that are fed onto the SiC surface15 do not necessarily have to form one layer. Less amounts of Al atoms17, such as an amount corresponding to 1/3 monolayer, for example, maybe used in coating the SiC surface 15 in a substantially uniform manner,and thereafter the rf-plasma excited active nitrogen 21 may be fed. (Inthe c-plane of a hexagonal crystal, a 1/3 or a single monolayercorresponds to regular adsorption.)

Hereafter, a crystal growing method according to a third embodiment ofthe invention is described with reference to the drawings. FIG. 7 showsthe steps of the crystal growing method according to the presentembodiment, and FIG. 8 shows a sequence chart regarding the timing offeeding of growth elements (which is controlled by turning on and off ofa shutter, for example, in the case of MBE) before and after the crystalgrowth of a Group III nitride.

In the following, the crystal growth steps in accordance with thepresent embodiment will be described with reference to FIGS. 7 and 8. Asshown in FIG. 7(A), initially a washed 4H—SiC(0001)_(Si) substrate 21 isprepared. Then a clean surface 25 is formed on the surface of thesubstrate 21 in the same manner as in the first and second embodiments.Thereafter, as shown in FIG. 7(B) and FIG. 8, at a certain time (timet20), the Ga irradiation is first carried out. Near the growthtemperature of AlN, the vapor pressure of Ga is higher than the vaporpressure of Al, so that the components of the Ga atoms that are adsorbedon the surface 25 of the SiC substrate 21 and the components that areevaporated from the surface are substantially equal, whereby a kind ofequilibrium is achieved. As a result, a state 25 a is created in whichGa in a state of equilibrium is adsorbed on the surface.

The growth of AlN is initiated by feeding Al and N simultaneously at acertain time (time t22), as shown in FIG. 7(C) and FIG. 8. Because theGa irradiation is performed for the purpose of controlling the mode ofAlN crystal growth at the beginning thereof, the feeding of Ga isterminated after the beginning of AlN growth at time t24.

Because Al and Ga are fed during the period t24 and t22, anAl_(x)Ga_(1-x)N layer 25 b is formed at the interface of the AlN layer35 and the SiC substrate 21, as shown in FIG. 7(D). If the presence ofan AlGaN layer between SiC and AlN is not preferable for the givenpurpose, such as when an AlN/SiC MIS structure is to be prepared, itwould be necessary to reduce the period t24 to t22 to be sufficientlyshorter than the growth time for a monolayer of the AlGaN layer. Forexample, if the Ga irradiation were to be stopped simultaneously withthe start of feeding of Al and N (t22=t24), there would be only a minuteamount of Ga at the interface, and the formation of the AlGaN layercould be virtually disregarded. On the other hand, if there are norestrictions as to the SiC/AlN interface structure, such as when abuffer layer for a GaN laser structure is to be prepared, the timing offeeding of Al can be given a certain degree of freedom. Specifically, Alcould be fed at t21 prior to the feeding of N at t22, or Al could be fedat t23 following the feeding of N at t22. If feeding were to be startedat t21, excess Al could be aggregated on the SiC surface if thepreceding irradiation time t22 to t21 is too long, resulting in a poorcrystallinity. On the other hand, if feeding were to be started at t23,a GaN layer would be formed because Ga and N would be fed during t23 andt21, whereby a SiC/GaN/AlN structure would be formed. After the AlNlayer with a desired thickness is formed, the feeding of Al and N isterminated so as to terminate the growth of the AlN layer.

By thus the irradiation of Ga atoms 27, which re-evaporate and do notbecome aggregated, first, the formation of an SiN film or excess Si—Nbond can be prevented and the mode of AlN crystal growth can be easilyrendered into the layer-to-layer growth, even without carrying out thepreceding irradiation of the Al atoms 31. In the method according to thesecond embodiment of the invention, adjustment of the timing ofpreceding irradiation of Al was required so as to prevent thedeterioration of crystallinity due to the aggregation of Al by excess Alirradiation, or the formation of an SiN film due to the lack of Alirradiation. In contrast, in the present embodiment, where the Ga atoms27, which are more easily evaporated than Al, are used, and theequilibrium state between adsorption and separation of the Ga atoms isutilized, such timing adjustment is advantageously not required.

Instead of Ga, In, which is similarly easily re-evaporated, may be used.In this case, too, the possibility of the rf-plasma excited activenitrogen 33 directly reacting with the SiC surface 25 can be reduced, sothat the formation of a SiN layer or the like on the SiC surface 25 canbe prevented and, as a result, the state of interface between the SiCsurface 25 and the AlN layer 35 can be maintained in a good condition.The feeding of N may be carried out in an intermittent manner. When thegrowth method involves a gas source for feeding Ga, such as in the caseof CBE (chemical beam epitaxy) or MOVPE (metal organic vapor phaseepitaxy), an organic metal containing Ga, such as trimethyl gallium ortriethyl gallium, would be fed, thereby substantially feeding Ga. Thesame applies to N, namely, by feeding a nitrogen-containing gas, such asammonia or hydrazine, N is substantially fed.

Hereafter, a crystal growing method according to a fourth embodiment ofthe invention will be described with reference to FIG. 9. In the crystalgrowth method according to the present embodiment, HCl gas etching wascarried out (see FIG. 9(A)) in the same manner as in the crystal growingmethod according to the first embodiment. Subsequent processes werecharacteristically carried out in an atmosphere with a smaller oxygenpartial pressure than normal. For example, the oxygen partial pressurecan be reduced to approximately 0.1 Pa by substituting the gas inside aglobe box connected to the sample introducing portion of an MBEapparatus with a high-purity inert gas, such as argon or nitrogen. Asshown in FIG. 9(B), the surface 43 of a SiC substrate 41 is cleaned withan aqueous solution containing hydrofluoric acid. The SiC substrate 41is then introduced into the MBE sample-introducing portion without itcoming into contact with oxygen, and the portion is evacuated to a highdegree. Then, as shown in FIG. 9(C), under a high vacuum (P=10⁻⁶ to 10⁻⁸Pa) within the MBE apparatus, as in the crystal growing methodsaccording to the foregoing embodiments, an AlN film 51 is grown. In thiscase, it is also important to remove the oxygen dissolved in the aqueoussolution containing hydrofluoric acid. This can be achieved by placingthe aqueous solution in an environment without oxygen partial pressure,or in the globe box in the above case, for several hours. When thismethod is used, the number of cycles of the irradiation processinvolving Si atoms or Ga atoms shown in FIG. 1(C) and the subsequenthigh-temperature process can be at least reduced, or could be entirelyomitted, thereby advantageously simplifying the relevant steps.

FIG. 10 shows example structures grown by the crystal growing methodaccording to the first or third embodiment of the invention. In thestructure shown in FIG. 10(A), there are only Ga atoms on the ppm orderremaining between the SiC substrate 1 and the AlN layer 53. In thestructure shown in FIG. 10(B), there is formed a thin Al_(x)Ga_(1-x)Nlayer 55 (x=0 to 1) between the SiC substrate 51 and the AlN layer 53.This is a layer formed by the growth of the AlN layer following thefeeding of Ga as shown in FIG. 8. When In is used instead of Ga, Inatoms 58 remain between the SiC substrate 51 and the AlN layer 53, asshown in FIG. 10(C). If In on the percentage order is mixed, a thinAl_(x)In_(1-x)N layer 59 (x=0 to 1) is formed between the SiC substrate51 and the AlN layer 53, as shown in FIG. 10(D). This is a layer formedby the growth of the AlN layer following the feeding of Ga or In asshown in FIG. 8. If any of the illustrated structures is present, it canbe presumed that the crystal growing method of the invention has beenused.

While the first to fourth embodiments of the crystal growing processhave been described with reference to the growth of an AlN layer as theGroup III nitride, a GaN layer or an Al_(x)Ga_(1-x)N layer can also beformed by the same method. For example, in the case of growing GaN, Gacan be fed instead of Al. In the case of growing Al_(x)Ga_(1-x)N, Al andGa can be fed simultaneously.

FIG. 11 shows examples of device structures according to the crystalgrowing method according to any one of the first through thirdembodiments of the invention. FIG. 11(A) shows an example of thecross-sectional structure of an MISFET in which the band discontinuitybetween SiC and AlN is utilized and the two-dimensional electron gaslayer at the interface is used as a channel. FIG. 11(B) shows an energyband structure in the junction structure of SiC and AlN.

As shown in FIG. 11(A), a heterojunction MISFET comprises a substrate 61with a SiC surface; a high-quality AlN layer 66 grown on the substrate61 by the crystal growing technique according to the present embodiment;a gate electrode 68 formed on the AlN layer 66; a source layer 63 and adrain layer 65 that are formed on either side of the gate electrode 68within SiC and in which high concentrations of impurities are doped; asource electrode 67a formed for the source layer 63; and a drainelectrode 67b formed for the drain layer 65.

As shown in FIG. 11(B), between the AlN layer 66 (with a bandgap ofapproximately 6.2 eV) and the SiC (with a bandgap of approximately 3.4eV) surface, there is a large band offset. As a result, when the gateelectrode is formed on the AlN layer 66, the concentration oftwo-dimensional electron gas induced at the interface can be controlledby the voltage at the gate electrode, whereby transistor operation canbe realized.

An improved AlN crystallinity can be obtained in the above-describedAlN/SiC-based MISFET due to the use of the crystal growing technique ofthe present embodiment, whereby a high-performance MISFET with good gateinsulating property and high channel electron mobility can be realized.

In the following, an example where the crystal growing technique of thepresent embodiment is applied to an optical device will be describedwith reference to the drawings. FIG. 12 shows a cross section of a laserstructure to which the crystal growing technique has been applied. Asshown, a semiconductor laser employing the crystal growing technique ofthe invention comprises: a substrate 71 with a SiC surface; an AlNbuffer layer 73 formed on the substrate 71; an AlGaN (n-type) claddinglayer 75; a GaN/InGaN multiquantum well (MQW) structure 77; and an AlGaN(p) cladding layer 78. On top of the AlGaN (p) cladding layer 78, thereis formed a first electrode E1. On top of the AlGaN (n-type) claddinglayer 75, there is formed a second electrode E2. In this laserstructure, too, a good AlN crystal can be formed on the SiC substrate71, so that the characteristics of the laser operation prepared thereoncan be greatly improved, thus contributing to the reduction of thethreshold current density of laser and to the increase in the life ofthe device.

The structure shown in FIG. 12 can be modified by changing the SiC 71 toan n-type and making the AlN layer 73 to be thin enough that a tunnelcurrent can be expected, such as on the order of 10 nm or less. In sucha modification, current can be caused to flow vertically. In this case,the second electrode may be formed on a back surface E2′ of SiC 71,which would make it possible to omit the step for forming a mesastructure of a Group III nitride. Alternatively, in the structure shownin FIG. 12, SiC 71 may be changed to an n-type, and an n-type AlGaNlayer may be used as a buffer layer instead of the AlN layer 73, so thatcurrent can be caused to flow vertically.

FIG. 13 shows an example of the structure of a heterojunction bipolartransistor (HBT) manufactured by the crystal growing method of thepresent embodiment. As shown, the HBT has a vertically stacked structurethat comprises: a n-SiC substrate 81; a p-SiC layer 83 formed on thesubstrate; and a n⁺-GaN layer 85 formed on the p-SiC layer 83. Anemitter electrode 91 is formed for the n⁺-GaN layer 85, a base electrode93 is formed on the p-SiC layer 83, and a collector electrode 95 isformed on the back surface of the n-SiC substrate 81, thereby forming anHBT. In accordance with the crystal growing method of the presentembodiment, a good interface can be obtained between the n⁺-GaN (GroupIII nitride) and p-SiC, so that the interface recombination can beprevented and the current amplification factor β, which is an importantperformance indicator in HBT, can be increased.

As described above, in accordance with the crystal growing technique ofthe embodiments of the invention, a Group III-N layer can be formed onthe SiC surface through the layer-by-layer growth or step-flow growthimmediately after the start of the crystal growth. As a result, theintroduction of defects that would be caused in the case of an islandgrowth can be reduced, and a thin film can be grown highly accurately.Thus, improved properties can be obtained by applying the invention toelectronic devices utilizing the wide bandgap of SiC or tolight-emitting optical devices utilizing a Group III nitride.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood thatvarious changes in form and details can be made.

INDUSTRIAL APPLICABILITY

The crystal growing technique of the invention, whereby a Group IIInitride is formed on an SiC surface through a layer-by-layer growth or astep-flow growth, makes it possible to reduce the introduction ofdefects and to grow a thin film highly accurately. Therefore, improveddevice properties can be obtained by applying the invention to theproduction of electronic devices or optical devices in which the widebandgap of SiC or a Group III nitride is utilized.

1. A crystal growing method comprising the steps of: forming a step-terrace structure on a SiC surface and then removing an oxide film from the surface; and performing at least one cycle of a process of irradiation of Si or Ga under high vacuum and then heating, and then growing a Group III nitride.
 2. The crystal growing method according to claim 1, wherein the step of growing a Group III nitride is performed at a temperature lower than the temperature of the substrate during the heating step.
 3. A crystal growing method comprising the steps of: removing an oxide film on a surface and forming a flat and clean SiC surface; and growing a Group III nitride under high vacuum, wherein a Group III element of an amount corresponding to a single monolayer or of a smaller amount is fed onto said clean SiC surface before nitrogen is fed.
 4. A crystal growing method comprising the steps of: removing an oxide film on a surface and forming a flat and clean SiC surface; growing a Group III nitride of an amount corresponding to a single monolayer or of a smaller amount on said clean SiC surface under high vacuum, wherein a surface control element for controlling the mode of crystal growth of said Group III nitride on the SiC surface is fed first; and feeding a Group III element and nitrogen, followed by the termination of the feeding of said surface control element.
 5. The crystal growing method according to claim 4, wherein said surface control element is Ga or In.
 6. A crystal growing method comprising the steps of: controlling a SiC surface to acquire a step-terrace structure; and removing an oxide film on the surface using a solution containing fluorine in an atmosphere of reduced oxygen partial pressure under high vacuum while the step-terrace structure is maintained.
 7. The crystal growing method according to claim 1, wherein said SiC surface has an offset angle of 0 to 15° with respect to the (0001)S_(i) or (000-1)C plane.
 8. A stacked structure comprising: an SiC layer; a AlN layer; and Ga atoms or In atoms on the ppm order remaining between said SiC layer and said AlN layer.
 9. The crystal growing method according to claim 1, comprising the step of forming a step-terrace structure on said SiC surface and removing an oxide film on the surface, and the step of removing the oxide film on the surface and forming a flat and clean SiC surface, wherein the step of growing a Group-III nitride comprises feeding nitrogen after the Group III element has been fed.
 10. The crystal growing method according to claim 1, comprising the steps of removing an oxide film on the surface and forming a flat and clean SiC surface, wherein the steps of growing a Group-III nitride under high vacuum comprises the steps of: feeding, first, a surface control element for controlling the mode of crystal growth of said Group-III nitride on said SiC surface, feeding a group III element and nitrogen, followed by the termination of the feeding of said surface control element.
 11. The crystal growing method according to claim 1, wherein the step of removing the oxide film comprises removing an oxide film on the surface using a solution containing fluorine in an atmosphere of reduced oxygen partial pressure, and then growing a Group-III nitride.
 12. A heterojunction MISFET comprising: a SiC substrate; an AlN layer formed by the crystal growing method comprising the steps of: forming a step-terrace structure on a SiC surface and then removing an oxide film from the surface; and performing at least one cycle of a process of irradiation of Si or Ga under high vacuum and then heating, and then growing a Group III nitride, or wherein the step of growing a Group III nitride is performed at a temperature lower than the temperature of the substrate during the heating step; a gate electrode formed on said AlN layer; and a source and a drain formed on either side of said gate electrode.
 13. A heterojunction laser device comprising: a SiC substrate; an AlN buffer layer formed by the crystal growing method comprising the steps of: forming a step-terrace structure on a SiC surface and then removing an oxide film from the surface; and performing at least one cycle of a process of irradiation of Si or Ga under high vacuum and then heating, and then growing a Group III nitride, or wherein the step of growing a Group III nitride is performed at a temperature lower than the temperature of the substrate during the heating step; a first AlGaN cladding layer formed on said AlN layer; a GaN/InGaN miltiquantum well structure; and a second AlGaN cladding layer formed on said multiquantum well structure.
 14. The crystal growing method according to claim 2, wherein said SiC surface has an offset angle of 0 to 15° with respect to the (0001)S_(i) or (000-1)C plane.
 15. The crystal growing method according to claim 3, wherein said SiC surface has an offset angle of 0 to 15° with respect to the (0001)S_(i) or (000-1)C plane.
 16. The crystal growing method according to claim 4, wherein said SiC surface has an offset angle of 0 to 15° with respect to the (0001)S_(i) or (000-1)C plane.
 17. The crystal growing method according to claim 5, wherein said SiC surface has an offset angle of 0 to 15° with respect to the (0001)S_(i) or (000-1)C plane.
 18. The crystal growing method according to claim 6, wherein said SiC surface has an offset angle of 0 to 15° with respect to the (0001)S_(i) or (000-1)C plane.
 19. The crystal growing method according to claim 2, comprising the step of forming a step-terrace structure on said SiC surface and removing an oxide film on the surface, and the step of removing the oxide film on the surface and forming a flat and clean SiC surface, wherein the step of growing a Group-III nitride comprises feeding nitrogen after the Group III element has been fed.
 20. The crystal growing method according to claim 2, comprising the steps of removing an oxide film on the surface and forming a flat and clean SiC surface, wherein the steps of growing a Group-III nitride under high vacuum comprises the steps of: feeding, first, a surface control element for controlling the mode of crystal growth of said Group-III nitride on said SiC surface, feeding a group III element and nitrogen, followed by the termination of the feeding of said surface control element.
 21. The crystal growing method according to claim 2, wherein the step of removing the oxide film comprises removing an oxide film on the surface using a solution containing fluorine in an atmosphere of reduced oxygen partial pressure, and then growing a Group-III nitride. 